A phase change type optical disc as a sort of optical information recording media uses a material which causes reversible phase change between crystalline and amorphous states as a recording layer, so as to conduct recording and erasing of information by utilizing the nature of the recording layer. The phase change type optical disc is capable of recording new information while erasing preceding information (hereinafter referred to as "overwriting") by merely changing the power of an irradiated laser beam corresponding to new recording information.
As the recording material for the phase change type optical disc capable of overwriting, chalcogen alloys, for example, In--Se series alloys (Appl. Phys. Lett. vol. 50, page 667, 1987) or In--Sb--Te alloy (Appl. Phys. Lett. vol. 50, page 16, 1987) and Ge--Te--Sb alloy have been mainly employed. Recording and erasing on the phase change type optical disc are conducted as described below. That is, upon recording, a laser beam at a high power focused through a lens is at first irradiated to a recording layer to melt the recording layer by raising the temperature to higher than a melting point (about 500 to 600.degree. C.). Successively, the recording layer is quenched by lowering the power of the laser beam. As a result of such rapid heating and quenching, the recording layer becomes amorphous and is in a stable state at a room temperature. An amorphous portion, for example, of about 1 .mu.m is thus formed on the recording layer (recording mark).
Upon erasing, a laser beam at a low power is at first irradiated to the amorphous portion to heat the same to a crystallizing temperature lower than the melting point. Subsequently, the amorphous portion is crystallized by gradual cooling to erase the recording mark.
As described above, since the laser beam at a high power is irradiated to the phase change type optical disc upon recording, a protection layer of high thermal resistance is disposed to both or either one of surfaces of the recording layer in order to prevent thermal damage (for example, deformation of a substrate, oxidation of a recording layer and movement of material along guide grooves).
As the material for the protection layer, materials comprising at least one of oxides, carbides, fluorides, sulfides and nitrides of metals or semi-metals are used. For example, Japanese Patent Laid-Open publication SHO No. 63-103453 proposes use of a mixture comprising, for example, ZnS and SiO.sub.2 as the material for the protection layer. It has been known that the protection layer comprising the mixture has higher thermal resistance than the protection layer only consisting of ZnS.
Further, a phase change type optical disc having high recording and erasing characteristics, which is also excellent in overwrite cyclability has been obtained by using a mixture comprising ZnS and SiO.sub.2 as the material for the protection layer and optimizing the thickness for each of the layers.
However, since the protection layer present on both surfaces of the recording layer is also repeatedly heated rapidly and quenched by overwriting cycles, there is a danger that the protection layer itself is deformed as the number of cycles increases extremely. If the protection layer itself suffers from deformation as described above, a problem results in that the recording layer is damaged causing a deterioration of the recording and erasing characteristics.
In order to solve the foregoing problem, Japanese Patent Laid-Open publication HEI No. 3-263627 proposes a method of depositing a first protection layer (a protection layer disposed to the recording layer on the side of the substrate) and a second protection layer (a protection layer disposed to the recording layer on the side of the reflection layer) which layers comprise a mixture of ZnS and SiO.sub.2, by a sputtering method using a gas mixture comprising Ar, N.sub.2 and O.sub.2 as a sputtering gas. Japanese Patent Laid-Open publication HEI No. 6-4904 proposes a method of depositing a first protection layer and a second protection layer which layers comprise a mixture of ZnS and SiO.sub.2 by a sputtering method using a gas mixture comprising Ar, N.sub.2 and H.sub.2 as a sputtering gas.
Japanese Patent Laid-Open publication HEI No. 4-119886 proposes a method of incorporating nitrogen into a recording layer and also incorporating oxygen into a first protection layer and a second protection layer which layers comprise a mixture of ZnS and SiO.sub.2. It also proposes a method of depositing the first protection layer and the second protection layer by a sputtering method using a gas mixture comprising Ar and O.sub.2 as a sputtering gas.
However, even the method disclosed in each of the publications does not have the effect of sufficiently improving the overwrite cyclability of the phase change optical disc. When the overwrite cyclability is improved by the method described above, it results in a deficiency in that the reliability of the recorded data is deteriorated.
The subject of the present invention has been to overcome the noted problems in the prior art by improving the overwrite cyclability without deteriorating the reliability of the recorded data in an optical information recording medium of a structure having protection layers on both surfaces of the recording layer.